The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
As used herein the terms “pixel” or “detector pixel” refer to a detector element without regard to the associated electronics. As used herein the term “detector channel” refers to a detector pixel, e.g. the sensitive volume of a scintillation crystal in conjunction with its photodetector, e.g. a Silicon Photomultiplier (SiPM) and its associated front-end-electronics.
As used herein the term “cumulative saturation” refers to saturation that is not due to a short intensive light flash, but due to a less intensive but continuous light incident on the photodetector (for example, but not limited to, SiPM photodetectors or photomultiplier photodetectors). This cumulative saturation is a result of the recovery time of the photodetectors (for example, but not limited to, SiPM photodetectors or photomultiplier photodetectors).
X-ray security systems for the inspection of cargo and shipping containers typically use transmission radiographic techniques based on use an X-ray fan beam generated by a pulsed high-energy X-ray source 110, such as a linear accelerator, linac. FIG. 1 (prior art) depicts a cargo inspection system employing such a technique.
A fan-shaped beam of X-ray 116, emitted by a source, linac or betatron based, for example, is detected by elements of a detector array 118 distal to a target object, here a cargo container 20, in order to produce radiographic images of the target object 190. The particular contents of the object may be discriminated and characterized on the basis of the transmission of X-rays through the object and its detection by the detector array and its individual detector pixels. Signals from each of the detector pixels, suitably pre-processed, provide inputs to processors, where radiographic image of object and material characteristics are computed. The thickness of the material to be penetrated by the X-rays may exceed 400 mm of steel equivalent in some cases. To insure the required penetration, inspection systems typically use X-rays with a maximum energy of several MeV, currently up to about 9 MeV. X-rays in excess of 1 MeV are frequently referred to as hard X-rays or high-energy X-rays.
Information (such as mass absorption coefficient, effective atomic number Zeff, electron density, etc.) with respect to the material composition of the contents of objects may be obtained on the basis of the interaction of X-rays with the material, and, more particularly, by illuminating the material with X-ray beams having energy spectra with more than one distinct energy endpoint (peak energy), or by employing energy discriminating detectors. Dual energy methods of material discrimination are widely used in X-ray inspection systems for security control of cargo in checkpoints. Dual energy inspection is discussed in the following references, for example, which are incorporated herein by reference:                W. Neale, et al., “Material Identification using X-Rays”. U.S. Pat. No. 5,524,133. (1996).        V. Novikov, et al. “Dual energy method of material recognition in high energy introscopy systems”. International Workshop on Charged Particle Linear Accelerators: Problems of Atomic Science and Technology, pp. 93-95 (1999).        S. Ogorodnikov, et al. “Application of high-penetrating introscopy systems for recognition of materials”. Proceedings of EPAC 2000, Vienna, Austria. pp. 2583-2585.        S. Ogorodnikov and V. Petrunin. Processing of interlaced image in 4-10 MeV dual energy customs systems for material recognition. In: Physical review special topics—Accelerators and beams, Vol. 5, 104701 (2002), 11p.        P. Bjorkholm. “Dual energy radiation scanning of objects” International Patent WO 2005/084352 (2005).Prior art examples of the methods of high speed X-ray cargo inspection utilized Scintillation-Cherenkov detection approach was introduced in US Patent Application 2011/0163236 (by A. Arodzero), incorporated herein by reference.        
Another recently proposed method is intra-pulse multi-energy cargo inspection using the natural intra-pulse variations in X-ray spectrum from conventional linacs, [FIG. 4 of U.S. Pat. No. 8,457,274 by A. Arodzero et al]. The detector signal is separately acquired for multiple time intervals relative to the pulse onset, and processed to obtain values corresponding to multiple-energy analysis of the transmitted radiation. Due to the effect of cumulative saturation of photodetectors (PMT and SiPM) [S. Vinogradov, A. Arodzero, and R. C. Lanza. Performance of X-ray detectors with SiPM readout in accelerator-based cargo inspection systems. 2013 IEEE Nuclear Science Symposium and Medical Imaging Conference, Oct. 27-Nov. 2, 2013, Seoul, Korea. DOI: 10.1109/NSSMIC.2013.6829597], this method (U.S. Pat. No. 8,457,274) only allows material discrimination for a very limited range of material thicknesses. Furthermore, the lack of controllability of the energy variation precludes use in an adaptive technique.
Another prior art example of high speed X-ray cargo inspection, FIG. 2, is the multi-beam inspection method presented in US Patent Application 2013/0136230 (by A. Arodzero and M. Rommel).
Other newer techniques in cargo inspection attempting to provide material discrimination with a non-interlaced, single energy linac, have been proposed, [US Patent Application 2011/0096906 by W. Langeveld]. One such method, direct spectroscopy (Z-Spec), uses an array of small plastic scintillators for gamma-ray spectroscopy in order to provide information about the atomic number of the traversed material. However, this approach remains impractical due to the high count rates needed for imaging large cargo containers.
Another proposed method, noise spectroscopy [W. Langeveld, et al. Noise spectroscopy: Z determination by statistical count-rate analysis (Z-scan). NIM A, 652 (2011) 79-83], provides spectral information indirectly by analyzing statistical fluctuations in the transmitted X-ray signal. A bright, high-Z scintillation detector (LSO or LYSO) with vacuum PMT readout is necessary for this method. The primary drawback is the high cost of these scintillation materials.
There is a need for an adaptable inspection system able to overcome the foregoing deficiencies and provide enhanced material identification adaptable to variable characteristics of the cargo under inspection.